1. Field of the Invention
This invention relates to a porous polyperfluorocarbon membrane having a water wettable surface. More particularly, this invention relates to a porous polyperfluorocarbon membrane having a water wettable surface formed of a perfluorocarbon polymer including hydrophilic functional groups.
2. Description of the Prior Art
Polyperfluorocarbon membranes are useful in a wide variety of environments due to the chemical inertness of the membrane. By the term "polyperfluorocarbon" as used herein is meant homopolymers of a perfluorocarbon as well as polymers formed from more than one monomer at least one of which is a perfluorocarbon including copolymers or terpolymers or a polymeric blend of such homopolymers and/or polymers or the like. Examples of polyperfluorocarbons include polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene copolymer (FEP) and perfluoro-alkoxy polymer (PFA or MFA).
Porous membrane filters are utilized in a wide variety of environments to separate materials within a fluid stream. Membranes are formed from a solid polymeric matrix and have highly precisely controlled and measurable porosity, pore size and thickness. In use, the membrane filters generally are incorporated into a device such as a cartridge which, in turn, is adapted to be inserted within a fluid stream to effect removal of particles, microorganisms or a solute from liquids and gases.
To be useful, membrane filters must be resistant to the fluid being filtered so that it maintains its strength, porosity, chemical integrity and cleanliness. For example, in the manufacture of microelectronic circuits, membrane filters are used extensively to purify various process fluids to prevent contaminants from causing circuit failures. Fluid filtration or purification is usually carried out by passing the process fluid through the membrane filter under a differential pressure across the membrane which creates a zone of higher pressure on the upstream side of the membrane than on the downstream side. Thus, liquids being filtered in this fashion experience a pressure drop across the membrane filter. This pressure differential also results in the liquid on the upstream side having a higher level of dissolved gases than the liquid on the downstream side. This occurs because gases, such as air, have greater solubility in liquids at higher pressures than in fluids at lower pressures. As the liquid passes from the upstream side of the membrane filter to the downstream side, dissolved gases come out of solution in the membrane resulting in outgassing of the liquid. Outgassing of a liquid can also occur spontaneously without a pressure differential as long as the liquid contains dissolved gases and there is a driving force for the gases to come out of solution, such as nucleating sites on the surfaces of a membrane where gas pockets can form and grow. Outgassing liquids typically used in the manufacture of semiconductors and microelectronic devices usually include very high purity water, ozonated water, organic solvents such as alcohols, and others which are generally significantly chemically active, such as concentrated and aqueous acids or bases which can contain an oxidizer. These chemically active liquids require the use of a chemically inert filter to prevent membrane degradation. Membrane degradation leading to the chemical breakdown of the membrane composition usually results in extractable material which is released from the filter during use, thus compromising the purity, integrity and cleanliness of the fluid being filtered. Polyperfluoro-carbon-based membrane filters made from fluorine-containing polymers such as polytetrafluoroethylene, or PFA are commonly utilized in these applications. Fluorine-containing polymers are well known for their chemical inertness, or excellent resistance to chemical attack. One disadvantage of fluorine-containing polymers is that they are hydrophobic and therefore membranes made from such polymers are difficult to wet with aqueous fluids or other fluids which have surface tensions greater than the surface energy of the membrane. In order to wet the surface of a hydrophobic membrane with water or an aqueous fluid, it is current practice to first wet the surface with an organic solvent, followed by contact of the surface with a mixture of water and an organic solvent and then followed by contact with water or an aqueous fluid. Alternatively, hydrophobic membranes can be wet with H.sub.2 O under pressure. This process is time consuming, expensive and often ruptures the membrane. Moreover, this process does not ensure that a substantial portion of the pores in the membrane are completely intruded with water.
Another problem often encountered during the filtration of outgassing liquids with a hydrophobic membrane filter is that the membrane provides nucleating sites for dissolved gases to come out of solution under the driving force of the pressure differential, during the filtration process. Gases which come out of solution at these nucleating sites on the hydrophobic membrane surfaces, including the interior pore surfaces and the exterior or geometric surfaces, form gas pockets which adhere to the membrane. As these gas pockets grow in size due to continued outgassing, they begin to displace liquid from the pores of the membrane ultimately reducing the effective filtration area of the membrane. This phenomenon is usually referred to as dewetting of the membrane filter since the fluid-wetted, or fluid-filled portions of the membrane are gradually converted into fluid-nonwetted, or gas-filled portions where filtration ceases and which results in a reduction of the overall filtration efficiency of the filter.
In contrast, self wetting hydrophilic membranes are spontaneously wet upon contact with an aqueous liquid so that a treatment process for wetting its surface is not required. That is, no prior treatment with an organic solvent or pressure intrusion, or mechanical energy such as by stirring is required in order for the membrane surface to be wet with water. It has been proposed in U.S. patent application Ser. No. 08/848,809, filed May 1, 1997, which is incorporated herein by reference, to provide a process for modifying a surface of a porous membrane such as a polyperfluorocarbon membrane with a bound perfluorocarbon copolymer composition to render the entire surface non-dewetting. A porous membrane substrate is intimately contacted with a perfluorocarbon copolymer composition in a solvent or diluent. Excess perfluorocarbon copolymer composition is removed from the surface with a solvent or diluent for the copolymer. The solvent or diluent does not remove the perfluorocarbon copolymer composition bound to the membrane surface. The membrane having the copolymer composition bound to its surface then is heat treated to improve the bond between the membrane substrate and the surface modifying perfluorocarbon copolymer composition. The perfluorocarbon copolymer composition is utilized in concentrations and amounts so that the membrane surface is completely modified while avoiding substantial blocking or plugging of the membrane pores. Complete surface modification can be determined by staining with Methylene Blue dye. In order to wet the surface modified membrane with water, it is first necessary to prewet the membrane with an organic solvent such as isopropanol (IPA). Thus, the surface modified membranes are not directly wet with water. In addition, these membranes must be maintained in contact with water or an aqueous solution in order to prevent the membrane from drying out. If the membranes are allowed to become even partially dried, the dried portion of the membrane must be wet via the complex process of contact with organic solvent, then with a mixture of an organic solvent and water and then with water or an aqueous solution.
When modifying a membrane surface it is essential that the surface modification be effected without substantially reducing the membrane porosity. Thus, sufficient surface modifying composition must be applied to effect the desired modification without substantially plugging the pores of the membrane substrate. Thus, the membrane having its surface modified should retain sufficient porosity to permit its use as a filtration membrane. These criteria require than an intermediate amount of surface modifying compositions be applied to a membrane substrate and therefore differ from coating a solid substrate such as films, powders or fibers.
International Application NO. WO95/24976 published Sep. 21, 1995 discloses a process for forming a thin coating of an ion containing polymer such as a perfluorosulfonic acid ionomer on a substrate such as powders, fibers, fabrics or films formed of a fluorinated polymer such as PTFE. The coating is effected from an aqueous solution of the ionomer which can contain alcohol. When coating fibers or powder, it was found that it is necessary to add a solution of the ionomer to the powder or fibers rather than adding the powder or fibers to the solution in order to attain the desired surface modification. In addition, it was found that the resultant mixture of solution and powder or fibers must be subjected to high shear conditions to effect the desired modification of the powder or fibers surfaces. When processing a film, the film is first immersed in an alcohol-containing solution of the ionomer followed by the necessary step of immersing the thus-treated film with a solution of a salt or a strongly ionized acid. The salt or acid treating step is necessary in order to obtain strongly adherent coatings on the substrate. It is also disclosed that after the treated film has been dried, it is necessary to utilize stirring energy to rewet the treated film when it is immersed in an aqueous solution in order to effect rewetting of the film. There is no disclosure of producing a membrane product which is directly wettable with water after being dried.
U.S. Pat. No. 5,716,680 discloses a process for coating powders or fiber made of a fluorinated polymer such as PTFE with an ion containing polymer such as a perfluorosulfonic acid ionomer. Consistent with the disclosure of International Application NO. WO95/24976, it is disclosed that it is necessary to add a solution of the ionomer to the powder or fiber to the solution to obtain the desired surface modification and that high shear conditions must be used on the mixture of ionomer solution and powder or fibers to attain the desired surface modification. There is no disclosure of producing a membrane product which is directly wettable with water after being dried.
U.S. Pat. No. 4,470,859 to Benezra et al, discloses a process for modifying the surfaces of microporous substrates formed of a fluorocarbon such as polytetrafluoroethylene, with a coating of a perfluorocarbon polymer composition including hydrophilic functional groups from a solution of the polymer composition to render the surface of the membrane more water wettable. The perfluorocarbon polymer composition is dissolved in a nonaqueous solvent at elevated temperature. The membrane then is immersed into the solution which, in turn, is placed into a vacuum chamber. The pressure within the chamber then is reduced such as to approximately 150 millimeters of mercury (absolute) to remove air from within the filter. Thereafter, the pressure within the chamber is quickly returned to atmospheric pressure. This coating process is repeated to ensure, what is described by Benezra et al., complete solution penetration into the pores of the membrane. By proceeding in this manner, the membrane surfaces and the interior walls defining the interstices within the membrane are coated with the perfluorocarbon polymer composition. Following the coating step, the solvent is removed by evaporation using heat and vacuum, or the solvated perfluorocarbon polymer composition is precipitated with a substance in which the perfluorocarbon polymer composition is effectively insoluble. The solvents utilized to form the solution include halocarbon oil, perfluorooctanoic acid, decafluorobiphenyl, N-butylacetamide, and N, N-dimethylacetamide.
U.S. Pat. No. 4,433,082 and 4,453,991 disclose a process for forming solutions of a perfluorinated ion exchange polymer such as copolymers of tetrafluoroethylene and methyl perfluoro (4,7-dioxa-5-methyl-8-nonenoate) or perfluoro (3,6-dioxa-4-methyl-7-octene sulfonyl fluoride) utilizing organic solvents which are relatively innocuous as compared to the solvents utilized in the coating process set forth above. The perfluorinated ion exchange polymers are dissolved in alcoholic solvents such as isopropanol at elevated temperature and pressure. The solutions obtained are disclosed as being useful in making and repairing films and non-porous membranes used in electrolytic processes such as aqueous sodium chloride electrolysis, in coating substrates such as catalyst supports for use in promoting a wide variety of chemical reactions, for coating porous diaphragms to convert them into non-porous articles and in recovering used perfluorinated polymers having sulfonic acid or sulfonate functional groups for reuse. In electrolytic processes, such as disclosed by these patents, extractables derived from the coated diaphragms are not a substantial concern and porosity of the modified diaphragm is disadvantageous.
Solutions of sulfonyl fluoride-containing fluoropolymers are also disclosed in U.S. Pat. No. 4,348,310. The solvents utilized therein are completely halogenated, saturated hydrocarbons, preferably having at least one terminal sulfonyl fluoride polar group. The solutions are disclosed as being used to repair holes in membranes made from fluorinated polymers and for making ion exchange film membranes, dialysis membranes, ultrafiltration and microfiltration membranes. Another disclosed use for these solutions is to coat porous diaphragms for electrochemical cells by contacting a diaphragm with the solution followed by evaporating the halogenated solvent and then hydrolyzing the coated diaphragm to convert the sulfonyl fluoride groups to the acid or salt form.
U.S. Pat. No. 4,902,308 to Mallouk et al, also describes a process for modifying the surface of a porous, expanded polytetrafluoroethylene membrane with a perfluoro-cation exchange polymer from an organic solution of the polymer.
U.S. Pat. No. 4,259,226 and 4,327,010 disclose modifying a porous membrane surface with a fluorinated polymer having carboxylic acid salt groups. No process steps are disclosed for controlling extractables from the membrane or for controlling the extent of binding of the modifying composition to the membrane surface. U.S. Pat. No. 5,183,545 and 5,094,895 disclose a process for making a multilayer, composite, porous diaphragm from a porous, multilayer, expanded polytetrafluoroethylene substrate having its surface modified with a perfluoro ion exchange polymer composition. The modifying polymer composition can contain a surfactant and may contain excess modifying composition, both of which are sources of undesirable extractables.
Polymeric surfaces, such as polymeric membrane surfaces modified with a perfluorocarbon polymer composition including hydrophilic functional groups and derived from an organic solution of the perfluorocarbon polymer composition are not directly wettable with water. In order to wet these modified surfaces with water, it is first necessary to intimately contact the modified surface with an organic solvent such as an alcoholic composition which is miscible with water. The modified surface and organic solvent then are intimately contacted with a mixture of water and the organic solvent and finally washed with water. By operating in this manner, the water penetrates the pores of the membrane and renders the membrane useful for passing aqueous solutions therethrough, such as during a filtration process. A characteristic of these water-wet modified surfaces is that they must remain in contact with water in order to render them useful for passing aqueous compositions therethrough. Should these modified surfaces become dry, they must be rewet with an organic solvent and water as described above since these membrane surfaces are not inherently water-wettable. This is undesirable since this rewetting process requires downtime of the filtration process and is therefore, undesirably expensive. In addition, the use of the organic solvent provides an undesirable source of contamination. In view of the non-self wetting properties of these modified membrane surfaces, the membranes or devices utilizing the membranes are shipped immersed in water or, when sold dry, require the customer to perform the wetting process described above.
Accordingly, it would be desirable to provide a porous membrane having its entire surface modified with a perfluorocarbon polymer composition including hydrophilic functional groups which surface is directly wettable with water. That is, the surface is wet with water by direct contact with water while avoiding the necessity of prior contact with an organic solvent or pressure intrusion or of energy such as shearing force after being dried. In addition, it would be desirable to provide such a membrane which is highly resistant to chemical attack, such as a membrane formed of at least one fluorine-containing polymer membrane substrate.